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Heating, process and system of raising the temperature of an enclosed space for the primary purpose of ensuring the comfort of the occupants. By regulating the ambient temperature, heating also serves to maintain a building’s structural, mechanical, and electrical systems.

The earliest method of providing interior heating was an open fire. Such a source, along with related methods such as fireplaces, cast-iron stoves, and modern space heaters fueled by gas or electricity, is known as direct heating because the conversion of energy into heat takes place at the site to be heated. A more common form of heating in modern times is known as central, or indirect, heating. It consists of the conversion of energy to heat at a source outside of, apart from, or located within the site or sites to be heated; the resulting heat is conveyed to the site through a fluid medium such as air, water, or steam.

Except for the ancient Greeks and Romans, most cultures relied upon direct-heating methods. Wood was the earliest fuel used, though in places where only moderate warmth was needed, such as China, India, Japan, and the Mediterranean, charcoal (made from wood) was used because it produced much less smoke. The flue, or chimney, which was first a simple aperture in the centre of the roof and later rose directly from the fireplace, had appeared in Europe by the 13th century and effectively eliminated the fire’s smoke and fumes from the living space. Enclosed stoves appear to have been used first by the Chinese about 600 BC and eventually spread through Russia into northern Europe and from there to the Americas,

2. a medium conveyed in pipes or ducts for transferring the heat to the spaces to be heated

3. an emitting apparatus in those spaces for releasing the heat either by convection or radiation or both.

Forced-air distribution moves heated air into the space by a system of ducts and fans that produce pressure differentials. Radiant heating, by contrast, involves the direct transmission of heat from an emitter to the walls, ceiling, or floor of an enclosed space independent of the air temperature between them; the emitted heat sets up a convection cycle throughout the space, producing a uniformly warmed temperature within it.

Air temperature and the effects of solar radiation, relative humidity, and convection all influence the design of a heating system. An equally important consideration is the amount of physical activity that is anticipated in a particular setting. In a work atmosphere in which strenuous activity is the norm, the human body gives off more heat. In compensation, the air temperature is kept lower in order to allow the extra body heat to dissipate. An upper temperature limit of 24° C (75° F) is appropriate for sedentary workers and domestic living rooms, while a lower temperature limit of 13° C (55° F) is appropriate for persons doing heavy manual work.

In the combustion of fuel, carbon and hydrogen react with atmospheric oxygen to produce heat, which is transferred from the combustion chamber to a medium consisting of either air or water. The equipment is so arranged that the heated medium is constantly removed and replaced by a cooler supply—i.e., by circulation.

If air is the medium, the equipment is called a furnace, and if water is the medium, a boiler or water heater. The term “boiler” more correctly refers to a vessel in which steam is produced, and “water heater” to one in which water is heated and circulated below its boiling point.

Warm-Air Heating

Because of its low density, air carries less heat for shorter distances than do hot water or steam. The use of air as the primary heat conveyor is nevertheless the rule in American homes and offices, though there has been a growing preference for hot-water systems, which have been used in European countries for some time. The heat of the furnace is transferred to the air in ducts, which rise to rooms above where the hot air is emitted through registers. The warm air from a furnace, being lighter than the cooler air around it, can be carried by gravity in ducts to the rooms, and until about 1930 this was the usual method employed. But a gravity system requires ducts of rather large diameter (20–36 cm [8–14 inches]) in order to reduce air friction, and this resulted in the basement’s being filled with ductwork. Moreover, rooms distant from the furnace tended to be under heated, owing to the small pressure difference between the heated supply air and cooler air returning to the furnace. These difficulties were solved by the use of motor-driven fans, which can force the heated air through small, compact, rectangular ducts to the most distant rooms in a building. The heated air is introduced into individual rooms through registers, grilles, or diffusers of various types, including arrangements resembling baseboards along walls.

Hot-Water Heating

Water is especially favored for central-heating systems because its high density allows it to hold more heat and because its temperature can be regulated more easily. A hot-water heating system consists of the boiler and a system of pipes connected to radiators, piping, or other heat emitters located in rooms to be heated. The pipes, usually of steel or copper, feed hot water to radiators or convectors, which give up their heat to the room. The water, now cooled, is then returned to the boiler for reheating. Two important requirements of a hot-water system are

1. Provision to allow for the expansion of the water in the system, which fills the boiler, heat emitters, and piping

2. Means for allowing air to escape by a manually or automatically operated valve.

Steam Heating

Steam systems are those in which steam is generated, usually at less than 35 kilopascals (5 pounds per square inch) in the boiler, and the steam is led to the radiators through steel or copper pipes. The steam gives up its heat to the radiator and the radiator to the room, and the cooling of the steam condenses it to water.

The condensate is returned to the boiler either by gravity or by a pump. The air valve on each radiator is necessary to allow air to escape; otherwise it would prevent steam from entering the radiator. The high temperature (about 102° C [215° F]) of the steam inside the system makes it hard to control and requires frequent adjustments in its rate of input to the rooms. To perform most efficiently, steam systems require more apparatus than do hot-water or warm-air systems, and the radiators used are bulky and unattractive. As a result, warm air and hot water have generally replaced steam in the heating of homes built from the 1930s and ’40s.

Electric Heat

Electricity can also be used in central heating. Though generally more expensive than fossil fuels, its relatively high cost can be offset by the use of electric current when normal demand decreases, either at night or in the wintertime—i.e., when lighting, power, and air-conditioning demands are low and there is excess power capacity in regional or local electrical grids. The most common method of converting electricity to heat is by resistors, which become hot when an electric current is sent through them and meets resistance. The current is automatically activated by thermostats in the rooms to be heated. Resistors can be used to heat circulating air or water, or, in the form of baseboard convectors, they can directly heat the air along the walls of an individual room, establishing convective currents.

Heat Pump

Another method for heating with electricity involves the use of the heat pump. Every refrigeration machine is technically a heat pump, pumping heat from an area of lower temperature (normally the space to be cooled or refrigerated) to an area of higher temperature (normally, the outdoors). The refrigeration machine may be used to pump heat, in winter, from the outdoor air, or groundwater, or any other source of low-temperature heat, and deliver this heat at higher temperature to a space to be heated. Usually, the heat pump is designed to function as an air conditioner in summer, then to reverse and serve as a heat pump in winter.

A heat pump’s operations can be explained using the following example. The typical window-mounted air-conditioning unit has a heat-rejection unit (condenser) mounted outside. This unit discharges the heat removed by the indoor coil (evaporator) to the outside air. Therefore the evaporator subtracts heat from the residence and transfers it to the refrigerant gas, which is pumped to the outside condenser, where by means of a fan the heat is dissipated in the air outside. This cycle can be inverted: heat is subtracted from the outside air and is transferred via the refrigerant gas to the indoor coil (evaporator) and discharged into a residence’s ductwork by means of the evaporator fan. This is a basic heat-pump system. Where winter climates reach freezing temperatures, however, the system is limited by the freezing of the condenser (outdoor coil);. thus, heat pumps work best in mild climates with fairly warm winter temperatures. The complexity of their machinery also makes them uneconomical in many contexts.

Solar Energy

Solar energy frequently works on a storage basis, in which water coils placed beneath heat-absorbing panels collect the radiant heat of the sun. This water may then be stored in a tank for use in heating lines or to provide hot water for washing and bathing. Solar Heated Water is storage in Storage Tanks / Storage Calorifiers, then pumped to heat reject radiators for room heating. In other way, the heated water can be directly passed through heat exchangers to ensure constant heat out at the rooms.